Journal of Physical Chemistry, Vol.98, No.48, 12495-12500, 1994
Delayed Ionization from C-70 by Electron-Impact Excitation
Delayed ionization from electron impact excitation of the gas phase C-70 has been observed. The nature of this delayed ionization was studied using a supersonic molecular beam apparatus with a quadrupole mass spectrometer. The results indicate that the field-induced ionization of long-lived high-Rydberg (HR) states in the detector region makes a significant contribution to the delayed ionization. Measured delayed ionization signals as a function of electron impact energy reveal two distinct types of excitation in C-70 One type of excitation, previously not observed in C-60, is a weak tail in the low-energy range which extends to electron energies below 10 eV. The other type of excitation, similar to the one observed in C-60, shows a prominent peak at about 50 eV. We suggest that the Rydberg states involved may originate from HR states of C-70 With a ground and an excited ion core configuration, respectively. The Rydberg states with an excessive energy in the ion core can dissociate as well as undergo delayed ionization. if the electronic energy of some of these states couples strongly with the nuclear motions of the ion core, delayed ionization of these states is then similar to thermionic emission that has been previously described in the delayed ionization of fullerenes by multiphoton laser excitation. A simple model calculation demonstrates that the statistical emission mechanism may contribute to the delayed ionization in the quadrupole. However, this contribution should be very small in the detector region.
Keywords:HIGH RYDBERG STATES;THERMIONIC EMISSION;AROMATIC-MOLECULES;FULLERENES;DISSOCIATION;C-60;C60;DYNAMICS